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Re: [Phys-l] internal/external conservative/nonconservative forces!?!?



Hate to appear dumb - but what are "bonds"? Are they physical entities? Are they a euphemism for something else? Are they a scenario?

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of John Clement
Sent: Wednesday, December 15, 2010 5:10 PM
To: 'Forum for Physics Educators'
Subject: Re: [Phys-l] internal/external conservative/nonconservative
forces!?!?

You get energy by forming bonds, and it takes energy to stretch out
bonds,
so if you get energy by forming a bond the bond does not "contain"
energy.
If you associate energy with the system, then students are less likely
to
think that bonds "store energy". They are less likely to think that
breaking bonds gives you energy.

And the forces are all electromagnetic in chemistry, but how do you
transfer
the energy to the E_k of the individual molecules?? That is something
I
have never thought about with respect to forces, but using energy
considerations you know it happens.

Getting energy out of the system is a multistep process where you first
break bonds, and then make new bonds. So there is an energy barrier to
overcome.

The system concept is valuable because it promotes better thinking
about
bonds. It simply does not make sense to say something is in a
container
ready to be released when in reality you are taking it (energy) out
when
making the container (bond). Just try asking students whether energy
is
released or absorbed when a bond is broken, and I think you may be
surprised
at the answer.

John M. Clement
Houston, TX



If this energy shows up as KE of the byproducts, then I think a
student
could legitimately ask "What force accelerated these byproducts to
these
high speeds?" I don't see how associating energy with the system
versus
the bonds helps the understanding of the chemistry. Simply reciting a
scenario where PE decreases and KE increases does not produce
understanding at a basic level - it's simply substituting a model for
the
dynamics. (This is why I became frustrated with Chemistry as an
undergrad
- too much scenario in the introductory courses.)

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of John Clement
Sent: Wednesday, December 15, 2010 2:27 PM
To: 'Forum for Physics Educators'
Subject: Re: [Phys-l] internal/external
conservative/nonconservative
forces!?!?


My example of bonds is very important to understanding chemistry.
Because
students will tell you there is energy in bonds and when you break
them
you
get energy. So if you dissociate gasoline the student will tell
you
that
releases energy. The student will tell you the same thing about
the
Krebs
cycle. This misconception blocks understanding of chemical and
physical
reactions.

lo.edu/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l